Frequency-modulated continuous-wave (FMCW) lidars allow remote measurements of range and velocity of objects. For the measurements it is necessary to displace from 0 Hz a beat signal developed by mixing a received signal and a reference signal. Usually, electronic or electro-optic modulators are used for this displacement. These modulators develop two frequency sidebands and do not allow defining the sign of Doppler shift, which is necessary for an unambiguous measurement of velocity. Only a direct frequency shifter can be used if velocity measurement is needed. Currently, only an acousto-optic modulator (AOM) is known to be used for this purpose. The main disadvantages of using an AOM are increased complexity and only a small available frequency shift as described in Reference [1] below, which is incorporated herein by reference. Also, an AOM may not be appropriate for integration into some applications.
The progress in semiconductor lasers in the last decade has resulted in development of small-sized high power lasers which can be directly used in integrated optical circuits. In addition, a linear frequency sweep of these lasers can be achieved simply by direct modulation of their currents.
Various frequency-modulation patterns can be used in lidar applications. The most common frequency-modulation patterns are a linear chirp modulation and a triangular frequency modulation. However, only a triangular frequency modulation, in which the frequency is swept up and down in frequency, allows discrimination of Doppler shift by calculation of a difference between the frequencies that correspond to the positive and negative slopes of the modulation. This is why triangular modulation is usually used if the velocity of an object needs to be measured, as described in References [2] and [3] below, which are incorporated herein by reference. The main disadvantages of the triangular frequency modulation is that it is necessary to use modulators to displace the beat frequency from 0 Hz, and the measurements of range and Doppler speed are erroneous in the cases of small range to the object and/or its high speed as is mentioned in Reference [3] below, which is incorporated herein by reference. Modulators increase the noise in the measurements, and the last circumstance can result in ambiguities concerning range and Doppler velocity.
An electro-optic inphase/quadrature (I/Q) modulator has been used to eliminate an acousto-optic frequency shifter, as described in Reference [1]. The main disadvantage of using an electro-optic inphase/quadrature (I/Q) modulator is the quite complex opto-electronics needed for both the transmitted and received beams, which considerably increases the noise in measurements.